The present invention relates generally to optical phased arrays. More specifically, the present invention relates to optical phased arrays having a tailored real index, guided amplifier structure. Advantageously, methods of fabricating an optical phased array having a tailored real index, guided amplifier structure, and systems employing the optical phased array monolithic device (e.g., a coherent chip) also disclosed.
Semiconductor lasers are the fundamental building blocks in compact optic and optoelectronics devices. Formed from Group III-V semiconductors, semiconductor lasers emit laser light in response to electrical stimulation, i.e., as electrons relax back to lower energy states, they emit photons. Stated another way, one of the most significant developments in semiconductor technology in recent years has been the increased use of III-V materials such as gallium arsenide and indium phosphide, and their ternary and quaternary alloys such as indium-gallium-arsenide-phosphide, as the active materials of semiconductor devices. The band gap characteristics of such materials typically make them candidates for optoelectronic and photonic applications such as lasers, light emitting diodes and photodetectors. For integrated circuit use, their high electron mobility often makes them preferable to the more commonly used semiconductor, silicon.
Fabrication of such devices generally requires epitaxial growth of one or more layers on a single-crystal substrate. Epitaxial growth refers to a method of depositing a material on a substrate such that the crystal structure of the deposited material effectively constitutes an extension of the crystal structure of the substrate.
The three broad classes of methods for deposition by epitaxial growth are liquid phase epitaxy, vapor phase epitaxy and molecular beam epitaxy (MBE), which respectively involve deposition from a liquid source, a vapor source and a molecular beam. One particularly promising form of vapor phase epitaxy is a method for deposition from a gas including a metalorganic compound, i.e., metalorganic chemical vapor deposition (MOCVD). MOCVD processes make use of a reactor in which a heated substrate is exposed to a gaseous metalorganic compound containing one element of the epitaxial layer to be grown and a gaseous second compound containing another element of the desired epitaxial material. For example, to grow the III-V material gallium arsenide, one may use the metalorganic gas triethylgallium [(C2H5)3Ga] as the gallium source and arsine (AsH3) as the source of the group V component, arsenic. The gas mixture is typically injected axially at the top of a vertically extending reactor in which the substrate is mounted on a susceptor that is heated by a radio-frequency coil. The gases are exhausted from a tube at the end of the reactor opposite the input end. Recently, the use of selective area growth (SAG) epitaxy, sometimes referred to as selective area epitaxy (SAE) in the manufacture of optoelectronic components has increased chip functionality by increasing the integration of more components on a single device (e.g. beam expanded laser, electromodulated lasers.
High brightness semiconductor lasers of the type discussed above are generally single mode waveguide structures that are limited to a few hundred milliwatts. It will be appreciated that higher power laser devices and systems are desirable. However, prior efforts to increase the power of conventional semiconductor laser devices via a larger gain region have met with limited success. Many tapered semiconductor laser are designed as free expansion devices in a gain guided region with no control over the position of the beam waist. In the resultant device, as the carrier concentration increases with drive current, the anti-guiding effects in the waveguide force the beam waist to shift. Many of the devices exhibit an effective shift in the direction of propagation of the beam, which makes it very difficult to match the output beam to a downstream micro-optic element. This anti-guiding effect can cause the far-field mode to increase in divergence as well as steer the beam as a function of the drive current.
In an effort to alleviate or at least mitigate the latter problem, a phased array of flared (tapered) amplifiers fed by phase adjusters and a power splitter producing a single high power beam when the flared amplifier sections are aligned and closely spaced was proposed in a paper by M. S. Zediker et al. entitled “10-Amplifier Coherent Array Based on Active Integrated Optics.” In the proposed device, which is illustrated in FIG. 1, a monolithic structure 20 includes an injection port 22, for receiving a beam generated by, for example, a master oscillator (not shown), an active distribution network 24 comprising turning mirrors 24a and Y-branch sections 24b, phase modulators 26, tapered optical power amplifiers 28, and lateral beam spreading guides 30. It will be appreciated that the output of the device 20 consists of, for example, 10 beams, which can be collimated and combined by downstream optical elements (also not shown). It should be mentioned at this point that it was envisioned that all of the phase modulators will be employed to ensure that all of the output beams will be phase aligned irrespective of the optical path length associated with a respective one of the output beams. It will be appreciated that, while the paper explains some of the difficulties inherent in fabricating a phased array of flared amplifiers, particularly with respect to maintaining single mode operation in all of the amplifier regions of the device, the paper tacitly admits that a practical device was beyond the capability of existing fabrication techniques.
Other devices employing a tapered or flared amplifier, such as a master oscillator power amplifier (MOPA), which uses a distributed Bragg grating (DBG) to define a master oscillator while employing a tapered section of the waveguide as a power amplifier, have been proposed. For example, a device similar to that disclosed by the Zediker et al. paper (discussed above) is disclosed in U.S. Pat. No. 5,440,576 to Welch et al. As illustrated in FIG. 2, a monolithic device 10 includes a first portion containing a DBR master oscillator 12 having an active region for lightwave generation, which is bounded by a pair of distributed Bragg reflectors 14 and 16, receiving power via a contact 18 connected to wire 20, a second portion including a waveguide 22 and a power splitter network 24, a third portion including a plurality of phase adjusters 68, 70, 72 and 74, and a fourth portion including flared amplifiers 78, 80, 82 and 84. The '576 patent discloses that the desired “phasing” is achieved by interfering outputs of less than all of the elements in the array; each interference pattern is adjusted for maximum contrast using the phase modulator associated with a flared amplifier from which an interfering beam portion emanates.
However, in disclosing this device, the '576 patent does not specify or even address the tailored index guide requirement needed to make the taper amplifiers work effectively, particularly at high power levels. Consequently, this design has the substantial shortcomings inherent in state-of-the-art devices at the time, i.e., circa 1994. Moreover, it will be appreciated that if the tapered amplifiers employed in the Welch et al. device have a constant index step, then the output power will be limited by the inability to maintain the single mode characteristics over the entire length of the taper. This would force the designer to either underconfine the mode in the narrow sections, or loosely confine the mode in the wider sections. If the mode is underconfined, then the propagation losses will be substantial and the power that reaches the power amplifier section will be insufficient to generate the desired output power. If the mode is loosely confined, then the anti-guide effects will be important, and the beam waist and far-field profile will be affected in the manner described above.
It should be mentioned here that all of the papers and patents mentioned herein are incorporated by reference. In particular, each of the patents mentioned by number is incorporated herein by reference in its entirety.
Accordingly, there is a need for an improved semiconductor amplifier structure. Stated another way, what is needed is a method for fabricating a tapered power amplifier having a corresponding tailored index profile suitable for ensuring single mode operation, and a stable beam waist and astigmatism over a broad range of drive currents. What is also needed is an optical phased array device having such a tailored index guided tapered amplifier structure. It would be beneficial if the device including an optical phased array having a tailored index guided tapered amplifier structure could be employed in an optical amplifier allowing signals from many tapered amplifiers to be coherently combined on a single optical fiber. It would be beneficial if the device including an optical phased array having a tailored index guided tapered amplifier structure permit coherent combination and steering of a far-field beam of advantageous profile through either a clear medium or a phase corrupting medium. Moreover, what is needed is an optical amplifier that minimizes the number of lossy elements employed in the network while minimizing the loss of signal-to-noise ratio through the device. Furthermore, it would be beneficial if the optical amplifier could be injection locked to a common optical signal with a plurality of other similar optical amplifiers, and arbitrarily phased to the optical signal such that the output beams from all of optical amplifiers advantageously can be coherently combined to form a far-field beam of advantageous shape even in the presence of an inhomogeneous index medium such as long paths through the atmosphere.